Syringe infusion pumps are often used to provide precise dosages of drugs injected for medical treatment via disposable syringes. They are especially effective for long-term injection of small volumes of solution where great accuracy is required, as the solution can be accurately delivered by precisely driving the plunger of a syringe down the syringe barrel at a continuous rate.
To achieve accurate flow rates and to determine the available and remaining volume of syringe, it is desirable to know the internal diameter of the syringe. In many syringe infusion pumps, user intervention is required to provide this information to the pump. For example, in some systems, the outside diameter of a syringe is measured with a linear potentiometer, and the system presents the user with a list of predetermined syringes known to have that outside diameter. The user then must either select or at least confirm the syringe type provided. Because the parameters of each syringe must be pre-programmed into the pump system, only a limited number of syringes are compatible with such a system.
Similarly, it is desirable to know the remaining distance a syringe plunger has to travel before the bung of the syringe meets the bottom of the syringe barrel. Some methods for determining the remaining plunger travel of a syringe have employed linear potentiometers to sense the position of the syringe plunger, based upon a user-selected hard-height for the particular syringe. These approaches are subject to error either from incorrectly selected syringes or from syringe variation (e.g., due to unacceptably large manufacturing tolerances). Still other approaches may measure the amount of force necessary to depress the plunger, and determine the syringe to be empty when the force exceeds a predetermined threshold. If the flow rate is low, however, such a system may result in long periods of non-delivery before the system determines the syringe to be empty.